A conventional prealignment sensor apparatus will be described with reference to FIG. 12. In FIG. 12, a table 4 is rotatably attached to the top of a shaft of the upper part of a motor 2, and can block a space between a light source 7 disposed at the lower part of its outer periphery and a CCD linear sensor 5 serving as a light receiving section disposed at the upper part of the apparatus when a wafer 1 is placed on the table 4. Reference numeral 26 designates a prealignment sensor, and is made up of the light source 7, a lens 8, the CCD linear sensor 5, a CCD linear sensor mounting board 6, and a frame 35 having a U-shape and holding them. Light of the light source 7 is changed into parallel rays of light by the lens 8, and is received by the CCD linear sensor 5. Reference numeral 10 designates a sensor controller, and is made up of a CCD linear sensor drive section 11, a wafer edge sensing section 12, a light emission drive section 13, a memory 14, a CPU 15, and a data communicating section 16. A system controller 17 is made up of a memory 18, a CPU 19, a data communicating section 20, an encoder signal processing section 21, a motor commander 22, a wafer presence sensor signal section 23, and a wafer transfer control section 24. The light emission drive section 13 feeds an electric current to the light source 7 so that light is emitted therefrom. The CCD linear sensor drive section 11 transmits a read-out-gate pulse signal (ROG signal), which is a timing signal used when the stored electric charge of pixels is transformed into an electrical signal, and a transfer pulse signal to the CCD linear sensor 5 consisting of a great many pixels that are linearly arranged and each of which has a fixed order, the stored electric charge is then read out in order from the first pixel that occupies a scan start point in accordance with the transfer pulse signal, the stored electric charge of all pixels is then sequentially output as a sense signal, and the wafer edge sensing section 12 receives the sense signal and other signals so as to sense the position. Information regarding the sensed position is output outward through the data communicating section 16, and the motor commander 22 of the system controller 17 outputs a rotation command signal to the motor 2, whereby the motor 2 is rotated. The wafer presence sensor 25, which is an optical, or a contact-type, or a capacitive sensor, is provided separately from the prealignment sensor 26, and can sense whether a wafer is present or absent in front thereof by allowing the wafer presence sensor signal processing section 23 to operate the wafer presence sensor 25. The encoder signal processing section 21 obtains the rotation signal of the encoder 3 connected to the motor 2 and senses the revolving speed of the motor 2.
A supplementary explanation will now be given of the CCD linear sensor 5. It is necessary to store an electric charge for the most suitable fixed time in the CCD linear sensor 5, in order to project a bright and dark image onto the CCD linear sensor 5 and generate a wafer edge signal. As a method for storing the electric charge for the most suitable fixed time, there are known, for example:    (1) a method for controlling the ON/OFF of light emission by the light emission drive section 13 synchronously with a measurement command and controlling the illuminating light 9 and the ROG signal so that a fixed amount of electric charge is stored in the CCD linear sensor 5 at each measurement,    (2) a method in which the CCD linear sensor mounting board 6 is provided with an electronic shutter function of the CCD linear sensor, and the illuminating light 9 is emitted constantly in quantity while light is always being emitted by the light emission drive section 13, thus controlling the storage time of the electric charge stored in the CCD linear sensor 5 independently of the ROG signal, and    (3) a method for keeping an electric charge stored in the CCD linear sensor 5 constant by allowing the CCD linear sensor drive section 11 to output an ROG signal and a transfer pulse signal at regular periodic intervals.
Method (1) has a problem in the fact that the repeated measurement cycles become slower, or the processing of the sensor controller 10 becomes complex, and method (2) has a problem in the fact that, since there is a need to provide the electronic shutter function of the CCD linear sensor, wires and costs increase, or the processing of the sensor controller 10 becomes complex. Therefore, conventionally, attention has been paid to improving the speed of prealignment time, facilitating the processing, and reducing the cost of the apparatus, and, as a result, method (3) has been widely employed.
With the aforementioned structure, the system controller 17 and the sensor controller 10 operate as follows. After a wafer conveying system, not shown, conveys a wafer to the table 4 when no wafer is placed on the table 4, the system controller 17 rotates the table 4 and allows the encoder signal processing section 21 to measure a signal of the encoder 3. When a predetermined rotational position is obtained, a measurement command is output to the sensor controller 10 through the data communicating section 20 to start measurement.
When the sensor controller 10 receives the output of the measurement command, the wafer edge sensing section 12 receives a wafer edge signal output by the CCD linear sensor 5, and a wafer edge sensed value is output to the system controller 17 through the data communicating section 16. The system controller 17 stores the received wafer edge sensed value and the measurement rotational position in the memory 18, and records outer-circumference data corresponding to one round of the wafer in the memory 18 by repeating the same operation until the wafer 1 makes one or more rotations. The center position, orientation flat, or notch position of the wafer 1 is calculated by the CPU 19 on the basis of the outer-circumference data corresponding to one round of the wafer recorded in the memory 18.
As a second conventional technique, a method for performing accurate positioning not by parallel rays of light but by a point light source is disclosed in Japanese Unexamined Patent Publication No. Hei-8-64660, and this conventional technique will be described with reference to a block diagram of a wafer position sensing apparatus of FIG. 13. A table 4 can rotate by a motor 2. A light source 7 is disposed at the lower part, and a CCD linear sensor 5 serving as a light receiving section is disposed at the upper part, with the wafer 1 placed on the table 4 therebetween. Light projected from the light source 7 to the outer circumference of the wafer 1 is shielded by the wafer 1, and a bright and dark image is projected onto the CCD linear sensor 5. This image is binarized by a signal-processing section 34b of a sensor controller 10c. A data value obtained at a moment when a change occurs from darkness to brightness is extracted from the binarized data, is then held by a latch, and is recorded in a memory 32b of a system controller 17c. This operation is repeated until the wafer 1 makes one rotation, and outer-circumference data corresponding to one round of the wafer 1 is recorded in the memory 32b. Simultaneously, a signal of the encoder 3 connected directly to the motor 2 by which the table 4 is rotated is input to the system controller 17c, and data regarding the motor rotational position and data regarding the wafer edge position are simultaneously recorded in the memory 32b. An arithmetic section 33b calculates the center position, orientation flat, and notch position of the wafer 1 on the basis of the outer-circumference data corresponding to one round of the wafer 1 recorded in the memory 32.
In semiconductor-fabrication equipment, a prealignment sensor for a wafer is conventionally used for the positioning of the center, orientation flat, and notch of the wafer. Therefore, a conventional wafer positioning method will be described with reference to a block diagram of an apparatus for sensing the edge position of a wafer outer circumference that uses the CCD linear sensor of FIG. 17. A substantially circular opaque wafer 142 is placed on a stage 141, and a light source 143 and a CCD linear sensor 144 are disposed with the outer circumference of the wafer 142 therebetween. When the outer circumference of the wafer 142 is illuminated with light emitted from the light source 143, the light is shielded by the wafer 142, and a bright and dark image is projected onto the CCD linear sensor 144. This image is used as an edge signal binarized by a signal-processing section 145 of a signal processing board 1411. A data value obtained at a moment when the edge signal is changed from brightness to darkness is held by a latch circuit disposed in the interior thereof, and is output and recorded in a memory 147 of a calculator 1410. The same operation is repeated until the wafer 142 makes one rotation by the stage 141, and outer-circumference data corresponding to one round of the wafer is recorded in the memory 147. The signal-processing section 145 and the memory 147 are operated according to a command of the CPU 149 of the calculator 1410 in coordination thereof. Based on the outer-circumference data of the memory 147, a data processing section 148 calculates the orientation flat position or notch position of the wafer 142 and the center position of the wafer 141. When this operation is performed, a scan starts from an end of the CCD linear sensor 144 in the signal-processing section 145. When a rise change point of the edge signal is sensed, edge position data obtained at this time is latched and stored in the memory 147. Normally, the edge signal reaches a state like an edge signal 1413 of FIG. 15, and a bright and dark change point of the image projected onto the CCD linear sensor 144 is regarded as a rise change point. The one scanning is repeatedly performed while the stage is making one rotation, and the center position, orientation flat, and notch position of the wafer 142 are calculated from data corresponding to one round thereof.
Next, a conventional prealignment sensor will be described with reference to FIG. 18 and FIG. 20. In FIG. 18, reference numeral 181 designates a frame whose side has a U-shape and has been fixed to a base, not shown. Reference numeral 182 designates a light source, which is an LED or a laser, fixed to the lower part of the frame 181. Reference numeral 183 designates a convex lens disposed at the lower part of the inside of the U-shaped frame 181 and by which diffused light is changed into parallel light. Reference numeral 184 designates a lens holder made of resin or aluminum and by which the convex lens 183 is fixed to the frame 181. Reference numeral 185 designates an optical receiver, such as a CCD linear sensor, disposed at the upper part of the inside of the U-shaped frame 181 and which has a sensing portion extending in a sensing direction in the Figure. Reference numeral 186 designates a signal processing circuit disposed at the upper part of the frame 181 and used to obtain the displacement magnitude of an object by processing an electrical signal output from the optical receiver 185. The light source 182, the convex lens 183, and the optical receiver 185 are disposed such that the respective center lines are aligned. The prealignment sensor consistes of the frame 181, the light source 182, the lens 183, the lens holder 184, the optical receiver 185, and the signal processing circuit 186. Reference numeral 187 designates a table disposed in the vicinity of the frame 181 and used to rotate a disk-like wafer 188 while placing it thereon. In FIG. 18, when the wafer 188 is placed on the table 187, the left end of the wafer 188 blocks a space between the convex lens 183 and the optical receiver 185.
With the aforementioned structure, the operation performed when the prealignment sensor senses the center position, orientation flat, or notch position of the wafer will be described as follows. Diffused light emitted from the light source 182 is first changed into parallel rays of light by the convex lens 183, and then projected onto the optical receiver 185. If the wafer 188 does not exist on the table 187 when projected, the parallel light is projected onto the whole surface of the sensing portion of the optical receiver 185. The wafer 188 is then placed on the table 187 so as to block the parallel light, and, as a result, a bright and dark image having a part resulting from blocking the light and a part resulting from transmitting the light is generated on the optical receiver 185. The bright and dark range thereof is sensed by the optical receiver 5, and is transformed into an electrical signal, thus making it possible to sense the edge position of the wafer 188. Further, if the edge position of the wafer 188 is sensed at a predetermined position while the table 187 is making one rotation in a θ direction, the center position of the wafer 188 can be calculated from the relationship between the rotational amount of the table 187 and the displacement.
Generally, aluminum subjected to alumite treatment for a metallic part and polyacetal resin for a resinous part are used as members constituting the prealignment sensor.
However, a conventional problem resides in that as the size of the apparatus increases, the cost thereof increases, and the number of wires increases when a wafer presence sensor is provided besides the prealignment sensor.
Another problem is as follows. If a method is employed for sensing a wafer edge by outputting a measurement command to the sensor controller 10 when the table 4 is rotated so that a rotational position obtained by processing the signal of the encoder 3 in the encoder signal processing section 21 becomes equal to a measurement position under the condition that the driving cycle of the CCD linear sensor is fixed, a measured wafer edge sensed value becomes a measurement value including irregular errors different from an original measurement position because of the asynchronous relationship between the measurement command and the driving cycle of the CCD linear sensor, and therefore, disadvantageously, difficulties arise in improving the speed of prealignment and in improving the accuracy thereof, thus exerting an influence on increasing the diameter of the wafer and on improving the throughput thereof.
Still another problem is as follows. It is known in the conventional technique that, since a wafer made of an opaque material like silicon is unsusceptible to dirt having the possibility of adhering to the CCD linear sensor, the CCD linear sensor should be scanned from a direction in which the wafer is inserted, and, in contrast, when a wafer made of a transparent material like glass is employed, only the edge part thereof blocks light, and therefore the CCD linear sensor should be scanned from a direction opposite to the direction in which the wafer is inserted. However, in the conventional wafer edge position sensor, the scanning direction of the CCD linear sensor is fixed, and, disadvantageously, the same wafer edge position sensor cannot use both the wafer made of an opaque material and the wafer made of a transparent material.
Additionally, in the conventional edge position sensing method mentioned above, if particles 1412 adhere to the CCD linear sensor 144 when the CCD is scanned in the direction opposite to the direction in which the wafer is inserted as shown in FIG. 15, the edge signal 1413 changes a plurality of times like the edge positions 1415 and 1416 of the particles, and, as a result, the first edge position 1416 is output, and the wafer edge position cannot be sensed correctly.
Additionally, in the conventional technique mentioned above, since the members constituting the prealignment sensor are unsuitable from the viewpoint of the purpose of use, gas is unfavorably emitted if the sensor is used in a vacuum, thus generating the cause of contaminating a vacuum environment. Additionally, since corrosion resistance is not high, problems arise when used in a chemical atmosphere. Therefore, difficulties lie in using the prealignment sensor in a vacuum or in a chemical atmosphere.
On the other hand, as shown in FIG. 21, light in the vicinity of a center axis of the diffused light admitted to the convex lens 183 from the light source 182 is reflected in the interior of the convex lens, and light is concentrated on the central part of the optical receiver. As a result, the luminous intensity level of the parallel light becomes uneven in the sensed range, and the received-light level of the optical receiver 185 varies, and, disadvantageously, sensing accuracy deteriorates.